The reprographic industry uses light-sensitive recording materials wherein differences in the tackiness of the exposed and unexposed areas are utilized for image production. For example, German Patents 12 10 321, 19 04 058, 19 04 059 and 20 04 214 disclose a reproduction process in which a tacky photopolymerizable recording material, consisting of a support and a photopolymerizable layer containing at least one addition-polymerizable monomer and a photopolymerization initiator, is hardened by imagewise exposure, whereby the exposed image areas lose their tackiness. The latent image is then visualized by applying a suitable toner, which adheres only to the unexposed tacky areas. Excess toner remaining on the exposed, non-tacky image areas is removed after application. This process produces positive, and optionally colored images of the original, and these images resemble in appearance images produced by the use of printing inks. Thus, the process has achieved considerable practical importance, particularly in the printing industry for proofing color separations.
Halftone color separations are used in reprography as copy originals for the preparation of offset or relief printing plates. The color separations are checked, prior to exposing the plate, with the aid of color proofing processes to determine whether the ultimate printing result will represent a tonally correct reproduction of the original. General standard specifications indicate that a resolution of 2%-98% dots at a screen spacing of 60 lines/cm is required for high quality offset printing. However, it is particularly difficult to achieve satisfactory resolution of pointy 2% dots in the highlights and of 98% dots in the shadows. It is also of considerable importance for screen dots of the same size to be sharply defined and uniform over the entire surface to achieve good tonal value reproduction.
The above requirements can be met using the transfer materials described in German Patent 36 25 014 which utilizes a special binder system of incompatible polymers. However, these transfer materials still show deficiencies and disadvantages, particularly relative to their preparation.
The process for preparing the transfer materials described in German Patent 36 25 014 has several disadvantages. It is (1) time-consuming and costly; (2) a controllable layer structure is quite difficult to achieve; (3) it requires as a prerequisite, the preparation and storage of two different incompatible polymers; (4) the pigments must be milled separately; and (5) the dispersions must be mixed intensively during the entire preparation process to prevent the components from settling and clumping. Only then can a uniform coating on the support be assured. The coating rate must also be low, and this adversely affects the production process.
German Patent 29 49 462 discloses, pigmented layers containing 75-95 parts by weight pigment and 5-25 parts by weight of an elastomeric binder. The process produces high opacity images that are used preferably for the reproduction of text and display pieces. However, due to the high opacity, the process is not suitable for color proofing processes. Reduction of pigment content does lead to a lower density but it also causes a complete loss of resolution. In addition, pigmented transfer materials must be sufficiently frangible to rupture imagewise during transfer. Thus, after removal of the layer, only the tacky image areas of the recording material are toned by the adhering layer. The pigmented transfer layers should not be too delicate or show gross ruptures because handling latitude is not assured and flawless image reproduction is not possible.
Transfer layers have been developed using core/shell polymers. For example, European Patent 01 07 378 describes protective layers for photographic materials. The protective layers are formed from a polymer layer of a latex composition consisting of core/shell polymers with 80-95 percent by weight of a core polymer with a glass transition temperature above 70.degree. C. and 5-20 percent by weight of a shell polymer with a glass transition temperature of 25.degree.-60.degree. C. The core/shell polymers described in Example 5 of European Patent 01 07 378 contain 40 percent by weight of a shell polymer having a glass transition temperature of 9.degree. C. These polymers form delicate layers that show gross ruptures even if the layers are cast separately. The transfer layers must also contain a pigment, so that the core/shell polymers yield delicate layers susceptible to flaws on handling.
The use of core shell polymers as binder additives, fillers or matting agents in photopolymerizable materials is described in European Patent Application A3 02 03 936. The application describes swellable microgels with glass transition temperatures above 25.degree. C. which are used to enhance the viscosity of photopolymerizable materials and to improve their storage stability. Core/shell polymers, among others, are described. The use of special core/shell polymers in pigmented transfer layers is not disclosed.
Accordingly, the object of the present invention is to provide a process for preparing images on tonable, light-sensitive layers by using a transfer layer which contains at least one toner. The process must be capable of achieving the resolution required by the printing industry. In addition, the process should provide highly uniform halftone values over the entire surface and avoid the previously described disadvantages and deficiencies.
It has been found that these objectives are achieved using special core/shell polymers for the preparation of pigmented transfer layers. Core/shell polymers comprising 5-95 percent by weight of a core polymer having a glass transition temperature above 50.degree. C. and 5-95 percent by weight of a shell polymer having a glass transition temperature below 40.degree. C. yield pigmented transfer materials of outstanding quality.